This invention relates generally to the field of communications and more particularly to underwater acoustic transducers and suspension systems therefor.
Passive underwater surveillance devices measure acoustic signals which are higher in level than the background noise. Generally, higher signal to noise ratios produce greater detection capability of the acoustic device. Background noise consists of a summation of ambient acoustic noise and selfnoise generated by the device, including flow noise caused by water flow around the hydrophone envelope and motion noise caused by the surrounding wave and flow environment physically moving the acoustic device. When the particular device being used has an analysis band that extends down into the infrasonic frequency range, the contribution of motion noise can be many times greater than that of other background noise and can impose severe limitations on systems performance. This dominance is caused by multiple ringing vibrations at each natural frequency of the underdamped mechanical subsystem suspending the hydrophone. These vibration effects are exhibited as spurious bursts of ringing energy that are generally unpredictable as to level and time of occurrence. Before a repeating transfer function relating the noise to the environment can be formulated, the suspension design must be simplified to eliminate the major portion of spurious noise effects. Only after such simplification can the effect of the remaining background noise on system performance be assessed. Generally, existing hydrophone suspension designs consist of a plurality of compliant members attached at the top of a cylindrical pressure or gradient hydrophone. Directional gradient hydrophones are much more sensitive to acceleration due to low frequency motion than pressure hydrophones, and rocking mode vibrations ranging as high as 5 Hz present a complex noise problem at the low infrasonic operational frequency range of 10 Hz to 50 Hz of such hydrophones. The electronics associated with the directional gradient hydrophone is sensitive to signals of acoustic or mechanical origin down to less than 1/2 Hz, but the system rolloff is such that below 10 Hz system gain decreases rapidly, and at 1 Hz extraneous noise due to vibrational motion does not appreciably affect hydrophone performance. It is to be understood that the other existing modes of vibration (i.e., vertical and horizontal translation, and rotational) do not present a significant noise problem to the hydrophone, because their respective frequencies are normally at or below 1 Hz.